Soil Organic Matter Quality in Chronosequences Of Secondary Northern Hardwood Forests In Western New England

We developed chronosequences in western New England northern hardwood forests for abandoned cultivated land (n = 10) and fields used as pasture or hay (n = 10). The top 20 cm of mineral soil was excavated using quantitative soil pits to examine particulate and potentially mineralizable organic matter as indicators of changes in soil organic matter (SOM) quality. The chronosequences were bound by land currently in agricultural use (n = 6) and native forest stands at least 250 yr in age with a history of minimal disturbance (n = 4). A 62-d incubation showed that a greater proportion of bulk soil C was potentially mineralizable in modern agricultural soils (108 g CO2-C kg(-1) soil C) than at forested sites (63 g CO2-C kg(-1) soil C), regardless of former use. Physical fractionation of SOM revealed that a greater proportion of bulk soil C was contained in the particulate organic matter (POM) fractions in plowed (86%) and pastured-hayed fields (49%) compared with regrowing forests 40 yr after abandonment (33-41%). Mineral-associated organic matter increased at a rate of 0.04 Mg ha(-1) yr(-1). While C/N ratios increased with forest age across the chronosequences for the bulk soil, they did not change with time for physical SOM fractions except for a weak but significant trend for the mineral-associated fraction of plowed soils (r(2) = 0.26, P < 0.01). Multivariate regression trees showed that differences in soil texture accounted for a significant portion of the variability in the mineral (250 mm) was better related to climatic variables. These results suggest that the stability of SOM increases when native forests are allowed to invade abandoned agricultural fields.